1. Field of the Invention
The present invention relates to an alkaline cell which comprises a negative electrode active material of zinc, which does not contain mercury (namely, a so-called "mercury-less cell"). Particularly, the present invention relates to an alkaline cell in which generation of hydrogen gas due to the presence of impurities in the cell, such as iron and nickel materials, is suppressed.
2. Description of the Related Art
In order to suppress generation of hydrogen gas due to corrosion of zinc within an alkaline cell in which zinc is used as the negative electrode active material, surfaces of particles made of zinc have been, hitherto, amalgamated with mercury. An amalgamation ratio as much as near 10% by weight based on the amalgamated zinc has been used so as to sufficiently suppress the corrosion of zinc. (See, for example, "Atarashii Denchi (New Cells)" published by Publishing Department of Tokyo Denki University (1978), p. 42).
However, environmental pollution due to the mercury of the cell has been emphasized nowadays and it has been required to reduce the amalgamation ratio.
Therefore, indium, lead, gallium, aluminum, bismuth and so on have been added to zinc instead of mercury to make an alloy with zinc so as to suppress the corrosion of zinc. Thus, the amalgamation ratio has been highly reduced. Therefore it is now required to use a cell without mercury (a "mercury-less cell").
The inventors have made studies on the cell without mercury, and found that the generation of hydrogen gas due to impurities contained in the cell is rapidly accelerated in the cell without mercury, which impurities have been of particular interest up to now.
That is, iron, nickel, chromium, an alkali metal, aluminum and so on are included as impurities in materials constituting the alkaline cell excluding an electrolytic solution. When zinc powder is put into and then mixed with the electrolytic solution containing such impurities, the following corrosion reactions of zinc are promoted, and thus the generation of hydrogen gas is also promoted:
(On anode side) Zn+4OH.sup.- .fwdarw.Zn(OH).sub.4.sup.2- +2e.sup.- PA1 (On cathode side) 2H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.- +H.sub.2
The impurities contained in the electrolytic solution originate from impurities contained in the potassium hydroxide for the preparation of the electrolytic solution and also from impurities contained in the cell constituting materials, in example the negative electrode material. It is impossible to completely exclude small amounts of such impurities contained in these materials when the cell is produced.
In the conventional cell, even when such impurities are contained, the generation of hydrogen gas is suppressed with the presence of mercury so that almost no problems occur. In conjunction with the "mercury-less cell", the problem of the rapid generation of hydrogen gas due to the presence of the impurities has been emphasized.